This invention relates to a method for forming a film of carbon added fluorine (fluorine-added carbon), which is for example used for an interlayer dielectric film of a semiconductor device.
In order to integrate semiconductor devices in higher density, various ideas have been proposed, for example, making patterns minute or multilayering circuits. As one of the ideas, there is known an art for multilayering wires. In order to achieve a multilayered wiring structure, an (n)-th wiring layer and an (n+1)-th wiring layer are connected with each other via an electric conducting layer. In addition, a thin film, which is called an interlayer dielectric film, is formed at an area or areas except the electric conducting layer.
An SiO2 film is a typical interlayer dielectric film. However, recently, it is required to lower a dielectric constant of the interlayer dielectric film in order to raise a speed of operation of the device further more. Then, characteristics of materials for the interlayer dielectric films are examined and studied. The SiO2 has a dielectric constant of about 4. Thus, various efforts have been made to find or form a material having a smaller dielectric constant. As one of the efforts, an SiOF film having a dielectric constant of 3.5 has been developed. However, inventors of this invention have paid attention to a film of fluorine-added carbon (hereafter, which is abbreviated as a CF film) having a further smaller dielectric constant.
If such a CF film is used as an interlayer dielectric film, for example as shown in FIG. 3, if such a CF film fills up a gap between aluminum wires 12, 12 of a circuit board which has a SiO2 film 11 on that the wires 12 are formed, a plasma unit is used for generating plasma, for example by using electron cyclotron resonance. In addition, for example, Ar gas is used as a plasma gas, and a mixed gas including a compound gas of carbon (C) and fluorine (F) and a hydrocarbon gas is used as a film-forming gas. Then, the film-forming gas is made into plasma to accumulate a CF film 13 on a surface of the SiO2 film 11.
While accumulating, as shown in FIG. 3, shoulder-like portions 14 of the accumulated film above both side edges of a concave portion 13 between the wires 12, 12 bulge out. Then, the shoulder-like portions 14 may contact with each other to block access to the concave portion 13. Thus, a bias electric power is applied to a stage for a wafer in order to draw plasma ions (Ar ions) perpendicularly toward the wafer. Thus, the portions blocking the access to the concave portion 13 are removed by a sputter-etching effect of the Ar ions while the film is formed.
In addition, in a real process to fill up the gap between the wires 12, 12 with the CF film according to the above method, the bias electric power for forming the CF film is divided into two stages in order to prevent generating voids. For example, the bias electric power in the first stage is 1.5 kW, the bias electric power in the second stage is 1.0 kW (which is smaller than that in the first stage), and other conditions to conduct the process are the same. In the case, in the first stage, since the bias electric power is larger, etching effect of the Ar ions is so strong that the portions blocking the access to the concave portion 13 are removed enough to fill up the concave portion 13 with the CF film. Then, when the concave portion 13 is filled up with the CF film, the first stage comes to an end. Then, the second stage starts. In the second stage, since the bias electric power is smaller, film-forming effect is stronger than etching effect. Thus, the CF film is formed at a higher speed in an area above the wires 12.
However, voids may be generated and good filling up may not be achieved if the CF film fills up a gap between wires having an aspect ratio (height of wires/gap between wires) of not less than 3, even if, according to the method described above, the etching effect and the film-forming effect are adjusted by the bias electric power in such a manner that the etching effect is stronger in the first stage and that the film-forming effect is stronger in the second stage.
This invention is intended to solve the above problem. The object of this invention is to provide a plasma film-forming method which can satisfactory fill up a concave portion having a high aspect ratio with a CF film as an insulating film.
In order to achieve the object, a plasma film-forming method of: making a film-forming gas including a compound gas of carbon and fluorine into plasma in a vacuum container including a stage for an object to be processed; and applying a bias electric power to the stage in order to draw ions in the plasma toward the object while forming an insulation film consisting of a film of fluorine-added carbon onto the object by the plasma; comprises: a first step of applying a first electric power of the bias electric power to the stage and supplying the compound gas of carbon and fluorine at a first flow rate to form the film of fluorine-added carbon onto the object; and a second step of applying a second electric power of the bias electric power smaller than the first electric power to the stage and supplying the compound gas of carbon and fluorine at a second flow rate smaller than the first flow rate to form the film of fluorine-added carbon onto the object.
Preferably, the film-forming gas includes a hydrocarbon gas.
Preferably, the film-forming gas is made into plasma by means of electron cyclotron resonance of a microwave and a magnetic field.
Preferably, the object has wires, the first step is conducted until the wires of the object are covered by the film of fluorine-added carbon, and the second step is conducted after the wires of the substrate are covered by the film of fluorine-added carbon.